The present invention resides in a method and apparatus for injection molding, especially the injection molding of large containers, in order to conveniently remove gas from the mold cavity and to stabilize the injection core. The present invention is particularly suitable for the injection molding of large containers, such as industrial containers, boxes, crates or large automotive container shaped parts, for example, heater chambers, gas tanks and the like.
Injection molding of large containers is conventionally performed using single cavity molds having one gate in the center of the base of the part. The gate diameter is usually made as large as possible in order to minimize shear heating of the resin flowing through the gate; however, too large a diameter gate causes the molding cycle to be extended in order to allow the large mass of resin at the gate to freeze off prior to mold opening.
In some cases, large diameter valve gates have been used with some success, without cycle penalty. Copending U.S. patent application Ser. No. 148,029 for LARGE NOZZLE FOR HOT RUNNER MOLD, By Paul Brown shows such a gate. However, single cavity valve gate systems extend the length of the flow path of the resin and increase the shutheight of the mold, see U.S. Pat. No. 4,449,915. In addition to the foregoing, the molten plastic flow path must pass around the valve stem and its control mechanism. This leads to additional complication in the molten plastic flow channel design.
A further solution to this problem is to use a machine nozzle containing a pin or valve shut off and extending the nozzle to reach directly to the mold gate. This overcomes the disadvantages of a single cavity valve gated mold, however at the expense of modifying the machine nozzle which may represent a considerable expense.
A further problem arising from gating large containers centrally as aforesaid is that considerable core shift can occur. This problem is well known especially in thin wall injection molding of any size container where the resin has a long flow path from the gate to the parting line. Several solutions to this problem have been proposed. U.S. Pat. No. 3,397,266 to Ayres shows a container with a flow restriction formed in the bottom of the part between the core and the cavity such that the core is stabilized by the filling action of the bottom panel before filling of the side walls commences. U.S. Pat. Nos. 4,743,420 to H. V. Dutt and 4,807,775 to Sorensen show methods of varying the wall thickness of the parts bottom using radial flow channels to preferentially fill the sides with a stabilizing rib of resin before completing the fill. U.S. Pat. No. 4,508,676 to Sorensen shows a two step approach with multiple gates wherein the first step is to mold stabilizing pads of resin from gates located along the side walls of the part and then to complete filling of the mold cavity by a second injection step from a central gate. This patent also shows a mold with moving inserts which are used to stabilize the core during filling and then moving the inserts away to allow completion of the fill. Copending U.S. patent application Ser. No. 221,026 for DIRECT GATING INTO THE LIP OF THIN WALLED CONTAINER, By Robert D. Schad and U.S. Pat. No. 4,622,002 to Bormuth show means for reducing core shift by supplying the resin from the lip of the part and filling toward the base.
The foregoing methods are not entirely successful for various applications for a variety of reasons, including cost, specifications for the particular part, complexity of design and other reasons.